Homoleptic organometallic compounds, which contain only one type of ligand bonded to a metal center (Zimmermann et al., Chem. Rev. 110:6194-6259 (2010); Edelmann et al., Chem. Rev. 102:1851-1896 (2002); Harder, S., Organometallics 21:3782-3787 (2002); Tsuboyama et al.; J. Am. Chem. Soc. 125:12971-12979 (2003); Wayda et al., J. Am. Chem. Soc. 100:7119-7121 (1978); Kruse, W., J. Organomet. Chem., 42:C39 (1972); Zucchini et al., J. Organomet. Chem. 26:357-372 (1971); Kleinhenz et al., Chem. Eur. J., 4:1687-1691 (1998)) have value in synthetic chemistry as catalysts (Watson et al., Acc. Chem. Res. 18:51-56 (1985); Kawaoka et al., Organometallics, 22:4630-4632 (2003); Barrett et al., Proc. R. Soc. A. 466:927-963 (2010)), as well-defined starting materials for single-site grafting onto supports for catalysis (Copéret et al., Angew. Chem. Int. Ed. 42:156-181 (2003); Quignard et al., J. Chem. Soc. Chem. Commun. 1589-1590 (1991); Quignard et al., Inorg. Chem., 31:928-930 (1992); J. Amor Nait Ajjou et al., Organometallics, 16:86-92 (1997)), as precursors for materials in chemical vapor deposition or other thermal decompositions processes (Valet et al., Chem Mater. 13:2135-2143 (2001); Edelmann, F. T., Chem. Soc. Rev. 38:2253-2268 (2009)), and for combination with a range of ancillary ligands as an entry-point into reactive organometallic compounds (Trifonov et al., Organometallics 20:4869-4874 (2001)). New homoleptic organometallics, thus, can lead to new possibilities in synthesis and catalysis.
Studies of homoleptic rare earth tris(alkyl) starting materials have typically focused on β-hydrogen-free alkyl ligands, namely CH2SiMe3 (Lappert et al., J. Chem. Soc. Chem. Commun. 126 (1973); Atwood et al., J. Chem. Soc. Chem. Commun. 140-142 (1978); Schumann et al., Anorg. Allg. Chem. 628:2422-2426 (2002)), CH(SiMe3)2 (Hitchcock et al., J. Chem. Soc. Chem. Commun. 1007-1009 (1988)), and CH2C6R5 (Wooles et al., Dalton Trans. 39:500-510 (2010); Bambirra et al., Organometallics 25:3454-3462 (2006); Huang et al., Organometallics 32:1379-1386 (2013); Bambirra et al., Organometallics 26:1014-1023 (2007)). Applications of homoleptic trivalent compounds containing these ligands, particularly those of the abundant light lanthanides (La, Ce, Pr, Nd), are limited by their thermal lability, challenging multistep syntheses, the formation of salt adducts, or the difficulty to exclude THF from the metal center's coordination sphere. For example, lanthanide tris(benzyl) compounds and their substituted derivatives are limited by the thermal lability of La(CH2Ph)3THF3 or Ce(CH2Ph)3THF3 at room temperature. Ligand design strategies have sought to overcome these difficulties.

For example, α-metalated N,N-dimethylbenzylamine lanthanide complexes are persistent at room temperature (Behrle et al., Organometallics 30:3915-3918 (2011)). Chelating ortho-dimethylaminobenzyl ligands also give stabilized organolanthanide complexes presumably due to intramolecular coordination (Harder, S., Organometallics 24:373-379 (2005)). The bulky alkyl ligand —C(SiMe3)3, provides homoleptic isolable, donor-solvent free compounds but is restricted to divalent Ln (II) compounds (Eaborn et al., J. Am. Chem. Soc. 116:12071-12072 (1994)). Interestingly, non-classical LnMe-Si interactions were observed in Yb{C(SiMe3)3}2 (Eaborn et al., J. Am. Chem. Soc. 116:12071-12072 (1994)) and La{CH(SiMe3)2}3 (Hitchcock et al., J. Chem. Soc. Chem. Commun. 1007-1009 (1988)). In addition, both of these donor-free homoleptic rare earth alkyls adopt solid-state structures that are distorted with respect to VSEPR predictions. Yb{C(SiMe3)3}2 is bent (C—Yb—C 137°), and La{CH(SiMe3)2}3 is pyramidal (ΣCLaC=330°), rather than pyramidal. The significant steric profile is the key to the persistence of these compounds.
The choice of alkyl ligand, however, may not need to be limited to the β-hydrogen-free hydrocarbyl groups. For example, [LntBu4]− and Cp2LutBu(THF) are isolable and eliminate isobutylene under only relatively forcing conditions (Schumann et al., Organometallics 3:69-74 (1984); Schumann et al., J. Organomet. Chem. 306:215-225 (1986); Noh et al., Polyhedron 26:3865-3870 (2007); Evans et al., J. Am. Chem. Soc. 104:2015-2017 (1982)). In catalysis, particularly ethylene polymerization, ultra-high molecular weight products are obtained from rare earth catalysts, and presumably the long polymer chains are accessible partly because β-hydrogen elimination is slow (Kempe, R., Chem. Eur. J. 13:2764-2773 (2007)). In such a scenario, the presence of β-hydrogen may stabilize reactive alkyl groups, as in Cp*2ScEt and other agostic compounds (Scherer et al., Angew. Chem. Int. Ed. 43:1782-1806 (2004), Burger et al., J. Am. Chem. Soc. 112:1566-1577 (1990)). Moreover, valuable aspects of metal-ligand bonding and reactivity is ignored in the absence of studies of β-hydrogen containing complexes.
An alternative means for stabilizing metal centers in homoleptic compounds, utilized mainly for amides, involves the β-silicon and β-hydrogen containing ligands such as tetramethyldisilazide —N(SiHMe2)2 and tert-butyl dimethylsilazide —N(tBu(SiHMe2) ligands (Rees Jr. et al., Angew. Chem. Int. Ed. Eng. 35:419-422 (1996)). Tetramethyldisilazide has been widely studied in d0 and f-element chemistry (Crozier et al., Chem. Commun. 49:87-89 (2013); Bienfait et al., Dalton Trans. 43:17324-17332 (2014); Anwander et al., J. Chem. Soc. Dalton Trans. 847-858 (1998)). Early metal and rare earth silazides containing β-Si—H often form agostic-type structures evident from low energy Si—H vibrations and deviation from Ln-N—Si angles within a given silylamide ligand (Crozier et al., Chem. Commun. 49:87-89 (2013); Bienfait et al., Dalton Trans. 43:17324-17332 (2014)). Ansa-lanthanidocene compounds containing N(SiHMe2)2 ligand exhibit an unusual β Si—H diagostic interactions (Eppinger et al., J. Am. Chem. Soc. 122:3080-3096 (2000)). Despite the rich chemistry of tetramethyldisilazido rare earth complexes, the chemistry of rare earth metals with β-SiH containing alkyl remains unexplored.
The chemistry explored thus far for ligands containing β-SiH groups is limited to the silazide. Previously, the synthesis of a β-Si—H containing tris(alkyl)yttrium complex Y{C(SiHMe2)3}3 (Yan et al., Chem. Commun. 656-658 (2009)) and bisalkyls M{C(SiHMe2)3}2THF2 (M=Ca, Yb)(Yan et al., J. Am. Chem. Soc. 131:15110-15111 (2009)) was demonstrated. These complexes contained non-classical β Si—H-M interactions, but they did not undergo β-H elimination upon thermolysis to 100° C. even though the metal center was (at least formally) coordinatively unsaturated. However, M{C(SiHMe2)3}2THF2 (M=Ca, Yb) reacted via β-hydrogen abstraction with Lewis acid.
The present invention is directed to overcoming these and other deficiencies in the art.